Accurate wellbore placement and surveying may be important for the development of subsurface oil & gas deposits. Directional drilling while sensing position and direction of drilling of the drill bit is known as measurement-while-drilling (“MWD”). Certain MWD systems use measurements of the Earth's gravitational and/or magnetic fields and may use gyroscopic measurement of rotational motion while drilling. These MWD tools may calculate well paths using inertial guidance. Such inertial guidance methods may allow the accumulation of error along the well path creating increasing uncertainties of position as the well is drilled.
Certain other methods allow for direct downhole determination of position in a borehole, which may result in the reduction of the uncertainty of the well path. One such method is active ranging, which uses the generation and measurement of magnetic fields. For instance, in certain traditional methods, a magnetic coil is placed downhole in a reference well. The magnetic coil is energized from the surface through a wireline to form a dipole magnetic field. The wireline, in addition to providing power, may also suspend and place the magnetic coil. The generated dipole magnetic field is measured by a triaxial magnetometer in another well being drilled, allowing the position of the drill bit to be determined relative to the reference well. Such traditional methods have been used for steam assisted gravity drainage (SAGD) extraction of heavy oil. Certain SAGD operations use two parallel wellbores, one above the other relative to the surface of the earth. These traditional coil systems methods may be limited in range and may require the use of wireline trucks to place and power the coil.
Certain SAGD operations have used single wire guidance methods. Single wire guidance systems use a magnetic field generated around a single current carrying wire. This wire may be placed on the surface or in a reference borehole, as described in U.S. Pat. No. 4,593,770, which is incorporated herein by reference. Because the magnetic field surrounding the linear wire of the single wire guidance method decays as 1/r, rather than the 1/r3 for the field surrounding a dipole, ranging at greater distances than with downhole coil active ranging is possible with single wire guidance.
In borehole single wire guidance systems, the reference well magnetic field may be dissipated because of a cancelling return current along the cased wellbore. Electrical isolation elements may be installed between adjacent casings to limit the return current through the pipe. In theory, nearly 100% of the current supplied down the wireline may be delivered to an insulated electrode section of casing in the casing string. The current may then enter the formation, and propagate through paths in the formation to a ground return rod on the current source. While this method may provide a strong signal and good ranging, like active coil ranging, single wire guidance may require wireline access to a well bore to place the single wire.